Page 21 in the original Underground Adventure book (Gemmell & Myers) gives a useful insight into what wasn't known for certain by the late 1940s, when Dalesman accepted the text. (Note that the book wasn't published until four years later.) Both Arthur Gemmell and Jack Myers were switched on professionals (architect & geophysicist respectively) and, if they'd have been aware of reliable dye tests by that stage, they'd no doubt have stated details. (This may help narrow down your literature search?) The information given in G & M mentions that their six inch Ordnance Survey map stated: "Issues at Douk Gill Plantation". Where the O.S. got that information from isn't known.
Beardy's post above gives a useful insight into the immense value of Cave Research Group publications. Very few present day cavers seem to be aware of just how good these are; they contain a vast amount of speleological knowledge and I'd recommend consulting them whenever a query such as the above is raised.